Why my object size has changed while appending?
The problem is solved. It appeared to be my mistake. When I complete the scene, I resized everything to make them fit in my view of camera because they were huge (the board's size was 48 Blender Units and the whole grid is 10 Blender Units as far as I know). So the newly appended objects were in correct size but every other object in my scene was too small compared to their original size.

... and didn't touch their size ...

Well... I think this doesn't count because I didn't resize them individually so I don't remember it while asking the question :D